The present invention involves a process that can be used to produce oxide ceramic powders for various electronic or optical applications. Some of the most commercially important oxide ceramics have the perovskite structure. Perovskite compounds have the general formula ABO.sub.3, where the A cation is relatively large and of low valence (such as Ba.sup.2+, Sr.sup.2+, Ca.sup.2+, Pb.sup.2+, La.sup.3+, Sm.sup.3+, Nd.sup.3+, Bi.sup.3+, K.sup.+, etc ) and the B cation is relatively small and of high valence (such as Ti.sup.4+, Zr.sup.4+, Sn.sup.4+, W.sup.6+, Nb.sup.5+, Ta.sup.5+, Fe.sup.3+, Mn.sup.3+, Mg.sup.2+, Zn.sup.2+, Ni.sup.2+, etc.). Perovskite ceramics have numerous commercial applications, mainly because of their useful electronic properties. These applications include materials for dielectric ceramics for capacitors; piezoelectric materials for transducers and sensors; electrostrictive ceramics for micropositioners and actuator devices; and transparent electro-optic ceramics for information storage and optical signal processing. A good discussion of perovskite structure is given in the June, 1988 issue of Scientific American, PEROVSKITES, pages 74 to 81, in an article by Robert M. Hazen.
The perovskite structure as typified by BaTiO.sub.3 above 135.degree. C. has a cubic structure. This structure consists of a regular array of oxygen ions at the corners, small tetravalent titanium ions in the center, and large, divalent barium ions located at the face centers. In ferroelectric perovskite compounds, the perovskite structure is distorted at low temperatures, and exhibits tetragonal, orthorhombic, or rhombohedral symmetry. At higher temperatures, the structure transforms to cubic; this transition temperature from the distorted phase to the cubic phase is called the Curie point. Ferroelectric behavior is caused by distortions in the crystal lattice caused by shifts in the position of the central cation (i.e., the Ti ion in BaTiO.sub.3); this results in a displacement of the centers of positive and negative charge of the ions within the structure and, thus, a net (or "spontaneous") polarization of the structure. The electrical properties are significantly affected by ferroelectricity in perovskites, giving rise to useful dielectric, piezoelectric, and electro-optic properties. The electrical properties of perovskites can be tailored to those required for a specific application by the wide range of compositional substitutions that are possible. The electrical properties of perovskite ceramics are also affected by manufacturing and processing conditions, as more fully described below.
The requirements of a powder for the numerous electric applications of perovskite ceramics depend on the specific material and its application. However, in most applications, the "ideal" powder is considered to have a fine particle size, narrow or no particle size distribution, chemical homogeneity, controlled stoichiometry, equiaxed particle shape, and to be agglomerate free. After a powder has been prepared, several processing steps are required to form the powder into a shape and to densify it into a finished functional electrical ceramic element. A powder is first formed or compacted into a partially dense shape called a green body. The exact shape depends on the electrical element's intended function and application, e.g., an electromechanical transducer or a multilayer ceramic capacitor. Once the powder is formed into a green body (e.g., by dry pressing or tape casting), the part must be densified by hot pressing, sintering, or the like. Sintering involves heating the green body to high temperature and allowing densification to occur by diffusional processes. The sintering conditions, e.g., time, temperature, pressure, and atmosphere, are dictated by the nature of the starting powder, the powder compaction, and the desired microstructure (e.g., grain size, microstructural uniformity and distribution of secondary phases) of the electrical ceramic elements. Some characteristics of the microstructure which can significantly affect the electrical properties of the ceramic element include grain size, and size distribution, amount and location of porosity, pore size and distribution, and controlled distribution of secondary phases. Sintering is a key aspect of the manufacturing process of ceramic elements and must be controllable to insure that the production of high quality ceramic materials is reproducible. However, reproducibility of the sintering process and the ceramic element is highly dependent on the reproducibility of the powder production.